1. Field of the Invention
This disclosure relates generally to hybrid analog-to-digital converters, and in particular, but not exclusively, those used in the readout circuitry of a complementary metal-oxide-semiconductor (CMOS) image sensor.
2. Background Information
Image sensors are ubiquitous. They are widely used in digital still cameras, digital video cameras, cellular phones, security cameras, medical devices, automobiles, and other applications.
Many image sensor applications benefit from an increased high dynamic range (HDR). One way to increase the resolution of ramp analog-to-digital converter (ADC) circuit is by increasing the counter resolution. This will increase the counter time, or the time required to convert analog output voltage from the pixel array into digital values used to construct a digital image which in turn will decrease the frame rate. For example, to increase a ramp ADC resolution from 10 bits to 13 bits will increase the counter time by eight times and may reduce the frame rate by almost one-eighth.
Another way to achieve an increased high dynamic range in a ramp ADC is by reducing the noise on the ramp generator. However, this will increase the power consumption and silicon area is required to increase the high dynamic range of an image sensor.